Alex's Cycle Blog

Friday, January 23, 2009

No, it's not a "someone made an inappropriate remark" story! It's another power meter, cycling training item. So for those non-PM using readers who's eyes roll around the top of their head when I go on about this stuff, then you can look away now :D .....

There have been lots of comments lately on the Google groups wattage forum about the Normalised Power (NP) algorithm and whether it could be improved. The discussion, as they often do, has drifted a bit from that into - "could the Training Stress Score (TSS) metric be improved?"

Well can they? Possibly.
Should we bother? I'm not so sure it matters.

Maybe it's because of the insensitivity of these things. Lemme show you an example.

I have produced a standard Performance Manager Chart (PMC). It covers my riding since I started back on the bike last year (~ 7 months).

As an experiment, I decided to add onto it another version of the PMC, with data based on an augmented TSS (TSS^). In this case, the calculation of TSS is not a function of the ratio of NP to Functional Threshold Power (FTP) but expressed as a ratio to Maximal Aerobic Power (MAP).

Now there is no particular reason for doing it other than curiosity, nor would there be any great sense given the underlying physiological and other rationale for choosing FTP as the anchor point. But that's not my point. It's an experiment to see what it means, from the point of view of how we actually use the information to monitor and guide our training.
MAP for most people is typically 25% +/-3% higher than FTP and so by anchoring an augmented TSS calculation to MAP instead of FTP, that will of course change the way TSS is calculated (since now I get a much lower weighting for threshold work and have to exceed MAP for gains to be multiplied).

And the impact of changing the TSS calculation? Well that'll change the PMC and how we interpret our training, right? Well, maybe.

Here's the PMC chart with two sets of lines for ATL, CTL & TSB. Default time constants used. One is based on TSS, the other (right hand axis) is based on the augmented TSS, “TSS^”. As always, click on the pic for a closer look.

Anyway the fact that the augmented ATL^, CTL^ and TSB^ mimic the same patterns, just with different absolute values, should not be a surprise since there is a reasonably consistent relationship between FTP & MAP.

Of course the relationship between FTP & MAP does vary (which it has during the period in the above chart), and when it does there will be deviations (as can be seen in the different slopes of the CTL and CTL^ lines).

But even so, just look at how closely the TSB and TSB^ lines track each other. Yet I have changed the TSS weighting formula quite a bit by anchoring to MAP instead of to FTP.

So if I showed you those charts independently, and multiplied the right hand axis values by two, you simply would not know the difference and it certainly wouldn't provide any different or additional insight into what was going on with my training.

So what would a PMC look like using these other “improved” formula for NP and/or TSS? That's what I'd like to see. Can it really provide us with a better insight into what's going on with our training?

I suspect all the mucking about with alternative NP or TSS formula would do is simply produce slight variations in the PMC (maybe absolute numbers a little different here and there) but the underlying training patterns that emerge would be the same and the interpretation would be the same. And even if the patterns are different, we still have to look at them in the context of the composition of our training, rest of life factors etc just like we do now (or should do).

Basically the modelling is pretty insensitive.

But let's see some examples folks....

I'm always open to looking at things in different ways to help garner additional insight.

Funny, in the several years I've been posting about power meter stuff, I haven't mentioned QA. Yet it is one many funky tools to help explain some the differences in the physiological demands of different types of rides.

I don't really have to go into much detail to explain it, since it's already been done by Dr Andrew Coggan and you can read all about it here.

But the short version is that QA is useful for examining the neuromuscular demands of a ride. Essentially it plots pedal forces versus pedal speed (the combination of both equaling power) for each data point recorded by the power meter. In this way, we can not only see how much power we produced during a ride but also gain additional insight into how we produced that power.

There are a number of ways such a plot can be used (e.g. examining and/or comparing ride data with your maximal pedal force-pedal velocity relationship) but I'll leave that for another day.

- Plotted in little red and blue dots/circles are the AEPF and CPV for each second of power recorded by the power meter. The data is from the "on" parts of my intervals only, that is just the time I spent at the intended effort. There is 40-minutes of data for each group.

- The green curved line shows the point at which pedal forces and pedal speeds, when combined, equal my Functional Threshold Power.

- the vertical and horizonal purple lines delineates the quadrants and represent 90rpm (with a 175mm crank) and 167 Newtons (or the same as applying a force of ~ 17kg).

We plot AEPF and CPV, since from a neuromuscular point of view, what's important is both the force and speed of muscle constractions/movement. Investigating either AEPF or CPV in isolation from the other is a fairly pointless exercise. (Refer Pithy Power Proverb "Cadence is a Red Herring" - R. Chung).

We can only plot AEPF, since each point of power meter data covers one or more revolutions of the cranks, in other words, the average of the forces applied to the pedal for an entire rotation of the crank. What this doesn't show is the variability in forces applied around the various points in the crank's revolution. As we know though, the greatest forces are applied on the downstroke, and by a happy coincidence, the maximal force exerted on the downstroke by each leg is roughly double the AEPF*.

* post edit: it was pointed out to me by Robert Chung I had expressed this relationship incorrectly (I had said "the maximal force exerted by one leg is the roughly the same as AEPF") and made this correction to show what I originally said as well as what it should have said.

CPV is basically similar to pedalling cadence, so why don't I just show cadence instead, since most people can related to what 90 rpm is like? Well in this case, the crank length on each bike is different. On the road bike I have 175mm cranks and on T7 I have 170mm cranks. So at the same cadence, the CPV would be higher on the bike with longer cranks. Or for the same CPV, my cadence would be slower on the bike with longer cranks.

If however you were examining ride data from rides using the same length cranks, then certainly you could also show cadence.

OK, so what do we make of the plots of my TTIs?

Well the first thing is that the dots are quite tighly grouped near the centre of the chart, which is pretty typical for efforts of a time trial nature. Generally the flatter the terrain, the more tightly grouped the dots will appear for a quasi-steady state effort. This contrasts significantly to plots for track races, criteriums and rides over hillier terrain, where the dots are widely scattered around the chart. In rides like MTB, the technical nature of riding can see a rider bumping up towards their maximal AEPF-CPV curve quite frequently.

The next thing is how much more tightly grouped the blue dots (indoor trainer) are compared to the red dots from the outdoor ride in the park. This shows that while the average power from these efforts was very similar, there were still differences in how I produced that power in each case.

We can see that the dots are close the the green line (denoting a pedal force/speed combination at FTP) and that the effort, overall on average, was just below my FTP.

The red dots tend to parallel the shape of the green line, which is reflective of me seeking to maintain power within a desired band over slightly variable terrain (I think the total altitude change is ~ 16-18 metres over the course of a 3.8km loop, with a few ups 'n' downs along the way). My speed varied significantly with the terrain and my cadence varied as well, although not by as much as speed since I would change gear regularly.

So, when riding on a trainer, there is a tendency for the AEPF-CPV relationship to show more of a rifle like plot during such Threshold Tolerance effort, whereas outdoors on more variable terrain (and conditions) the plot looks a little more like it came from a shotgun, albeit it one with an odd shaped barrel!

Is it important?

Well it simply serves to show that similar efforts can have variable neuromuscular demands and even changes as small as this may affect the power one is capable of producing in a given scenario. It just emphasises the specificity principle.

If your time trials are outdoors, makes sure you do some time trial training outdoors and ensure your legs are ready for the more variable neuromuscular demands.

This morning was meant to be my Threshold Tolerance Intervals (TTIs) - the good ol' 2 x 20-min workout at near FTP. Target range at the moment for me is 91-96% of FTP (250-265W).

So I drive to park today, hop out to attach my leg and get bike ready, Sam was riding past and sees me so stops to say hello. I get my leg on and roll off, intending do a roll for a lap or two with Sam before getting into it. But of course after 3-min a spoke goes "ping" and that's was it, wheel not in a trainable condition unfortunately. Hop back in car and go home.

So I hop onto Thunderbird 7 instead.

Last week I did my TTIs in the park at 259W and 255W.
On T7 this morning: 258W and 266W.

I intend to do a Quadrant Analysis plot of the indoor vs. outdoor TTIs and post about that soon. I suspect we'll see the difference akin to the mark on a target made by a rifle and a gunshot.

Saturday, January 10, 2009

There's been a bit of discussion lately on various training forums about a topic that seems to crop up every so often. It's a perennial favourite. Certainly I'm not the first to write about it and I won't be the last.

Why is my power different when training indoors* compared to when I ride outside? And what can I do about it?

* Indoor training being training done on an ergobike, or with the bike locked into a turbo trainer or riding your bike on rollers. Often performed inside the house, in the garden shed or garage, on a balcony or at the local gym or training centre.

Usually people train in such a fashion because they either haven't the time or opportunity for a ride outdoors, they might be recovering from injury and/or need the controlled and safe environment an indoor trainer provides, or the riding conditions outside are not suitable (cold, rain, snow, darkness and so on). Certainly riding indoors is a safe and excellent training alternative when heading out the front door on your favourite steed is not possible.

For many riders though, they find generating power indoors much harder than when riding outdoors and end up riding at a lower power as that's all they can do (but this is not the case for all though, and some can actually produce more power indoors than outside, although that is less common).

So if I can't generate the same power, then am I getting the same training benefit?

And if power is significantly different indoors, should I use a different FTP for indoor rides (so training levels and ride data are adjusted accordingly)?

Well the answers are not straightforward but let's explore the solution(s).

The first thing to do is to understand why a difference in power production exists. Then the second thing is to take steps to address the differences between each scenario and "bridge the gap". Finally, one then needs to make decisions about how the data from their indoor training should be interpreted.

So why is it common for power to be different?

There are four main factors at play here:

Cooling & air flow

Inertial load

Motivation

Adaptation

I'll explore each of those in a bit more depth a bit further down.

OK, so what about the training benefit and setting of FTP?

Well power is power and if you are burning kJ at a lower rate, then the metabolic adaptations relating to that will be correspondingly different. So if turning out a lower power really concerns you, then the priority is to address the factors that influence indoor power production and reduce the gap so that training can still be done within the intended training level. Then the problem goes away.

Nevertheless, "hard is hard" and "alls you can do is alls you can do", so if you are unable to address/fix the key reasons why power is less indoors, then set your training at a level that is attainable for that scenario. It's better than staying on the couch. Rather than worry about what percentage of FTP or MAP that should be, just use previous indoor workouts as your guide. That really should be the guide anyway, irrespective of mode of training you are doing.

What matters is that you do the workout at around the right intensity for the right duration, rather than the precise wattage.

What about FTP, and the calculation of TSS and the other metrics that flow from it?

This really is an issue of what you are training for and where the majority of your riding will be during the course of that training period. If the trainer only represents a minority of your ride time and your power is say 10% less on the trainer, then it only represents a small difference in the calculation of overall training load. It is simply not worth the bother to have separate FTP values and calculations. The Impulse-Response model (aka the Performance Manager and the metrics CTL, ATL and TSB) is fairly robust. It is about the forest, not the trees.

For example, let's examine a common hour-long training ride and say, for whatever reason, your indoor power is 10% less than outdoors: 2 x 20-min at FTP + warm up and cool down.

Outdoors, this would accumulate ~ 85 TSS and indoors (with ~10% less power), ~ 70 TSS. A difference of 15 TSS (which is about the equivalent stress of 15 minutes of endurance level riding).

So if the difference in TSS calculated from a 2x20 workout (equivalent to about 15-minutes of basic endurance level riding) is concerning you, then sit on the trainer for another 15-minutes.

If however, the trainer represents (for that training period) a large proportion of your training time, then setting FTP according to that training mode makes sense. But where such rides are only occasional, then there really is no reason to worry about minor variations in the numbers, just move onto the next day.

The same principles apply if talking about training at altitude (occasional change in altitude vs. a lengthly block at a different altitude) or different bikes (occasional or lengthly training blocks on a given bike/position).

Read on for more details on the four elements of indoor training that affect our ability to produce power indoors and how you might do something about it.

CoolingPeople consistently underestimate the cooling needs when training indoors. There's some weird theory that a large pool of sweat forming beneath you is a good thing. All that tells me is that the air flow and cooling arrangement is perhaps inadequate for the task. A body that is under stress and not being adequately cooled will underperform.

Keep in mind that the typical cyclist operates at around 21-22% efficiency (give or take a couple of percentage points). Cycling efficiency is a measure of the ratio of energy reaching the cranks of the bicycle as a proportion of the total energy metabolised*. In other words, to generate 100W at the cranks, our bodies are metabolising energy at the rate of 100W / 21.5% = 465W.

So of that 465W, 100W is converted to mechanical energy at the cranks, with the vast majority of the balance being converted to (waste) heat, with a bit used of course to run the rest of the body's functions.

What that means is that for every 100W we put through the cranks, roughly another 360W are generated as waste heat. How much heat exactly will vary depending on the individual's efficiency level (typical range is 19-24%).

So if for instance you are doing some intervals at 300W, you are in effect pumping out the heat equivalent of a 1,000-1,200W electric heater! Now do you see why we heat up so quickly when training hard and an effective cooling system is required? Especially if the ambient temperature is quite warm to begin with, and particularly so if the conditions are humid.

When you hop on your bike for an endurance ride, you have a ~ 30km/h wind flowing over your whole body constantly wicking sweat away and keeping you cool(er). So why would you expect to perform as well indoors with no air flow, or the piddling excuse of a breeze that comes from a domestic fan? Get real. If training indoors is going to become a sizeable chunk of your training time, then get some decent cooling happening and have some strong air flow over you. A large industrial strength fan costs much less than a trainer or rollers, so bite the bullet and sort it. But be prepared for the additional noise.

Some people do perform indoor training in quite cold environments, so of course they might be able to get away with less air flow than others.

* there are a couple of different efficiency measures (e.g. gross and delta efficiency), but for all intents and purposes, this basic definition will suffice in this context.

Inertial load is the next main differential factor when comparing indoor and outdoor training. Without going into too much detail, when we ride outdoors, we have the inertial load of a bike and rider moving at some speed, plus that of the wheels turning. If we stopped pedaling, our rear wheel doesn't suddenly slow or stop turning, we would coast for quite some time. On many trainers however, since we are not moving, the inertial load is much less and confined to the rear wheel spinning and any small flywheel that the trainer has attached to the roller. When you stop pedaling, the wheel slows and comes to a halt relatively quickly. Some are worse than others.
Now what happens is each scenario feels quite different to ride, muscle activation is different, the neuromuscular demands are different and these can be enough for some to make power production much harder. In general, low inertial load trainers tends to emphasise the "dead spots" in the pedal stroke (when the cranks are passing through the 12/6 O'Clock position), whereas riding with a higher inertial load enables one to breeze through (and not waste effort on) the dead spots and focus on the downstroke where the bulk of power is produced.

Fortunately there is a way to increase the inertial load of a trainer, and that's by having a flywheel attached to the trainer's roller (or even by adding mass to the wheel itself). How much mass is needed? Well to replicate the inertial load of a rider, it would need a very heavy flywheel spinning very quickly. Think of a 20-30kg flywheel spinning at 500-800 rpm. Yikes!!

Fortunately, for effective training, going that far is not really necessary and having enough rotating mass to help smooth out those dead spots is enough. I don't have one myself but trainers like the Kurt Kinetic Road Machine or the 1-Up trainer are an excellent example of this. They both have small but effective flywheels attached to the rolling mechanism.

These are ideal options for those that are looking to attach their existing bike to a trainer but also need some portability with their indoor unit.

The other option is a dedicated ergobike like the Schwinn or Saris indoor trainers (or other similar machines). These types of set up have the advantage of being able to incorporate a much larger and heavier flywheel than a turbo trainer. They are of course dedicated units and need a permanent place to live.

So what does one do? Well if cooling has been sorted, and power is still down, then consider the inertial load of your trainer set up. Does it have a flywheel? Can one be added? Should I look at an alternative trainer? Certainly I would recommend trying a trainer that has a decent flywheel to see how much better it is to ride.Edit note: I added the following paragraph in March 2011 as something I'd been meaning to do for quite some time, just had forgotten to do it.I should add that the idea of inertial load on an indoor trainer affecting pedaling isn't actually backed by evidence other than anecdotal, from myself and many others I know that have used such trainers. As an example, this link to a study extract on PubMed indicates that varying crank inertial loads has little or no effect on steady state pedaling coordination.

Motivation is a big issue in training and racing, and it is sorely tested when riding indoors. Many find training indoors mind-numbingly, excrutiatingly boring. Then there are others who really love it and are happy to spend hours tapping it out, sometimes preferring that to a ride outdoors. Each to their own.

If a lack of motivation is an issue, then it needs to be addressed, otherwise don't waste your money on a trainer you won't use. It'll just end up gathering dust in the corner of the shed.

There are many ways to overcome any motivational challenges you face:

Variety - there are lots of training workouts available, so keep the variety up. Dream up some of your own!

Duration - indoor riding is hard work, there's no let up or coasting, so don't make the workouts as long as you might ride outdoors. It is better to complete a shorter workout and want to come back for more next time, than to get off absolutely hating it and sitting out the next one on the couch or staying in bed.

Set Challenges - set yourself targets for the session and maybe have reminders of your goal event in front of you as well.

Music - this is a good one - having you favourite training tunes blasting away, or on your iPod to keep the neighbours happy.

Video - what about watching highlights of your favourite stage race or one-day classic. You can be smacking it up Ventoux with the Pros. Of course there is a big market out there for indoor cycle training videos, so if that floats your boat, then go for it!

Computer aids - there are lots and some of the favourites are heart rate monitors, spped and cadence measurement computers and of course my favourite - power meters. These are especially helpful so that training is focussed and performed at the right intensity.

Ergo controllers and virtual riding - there are many trainers that can automatically control the resistance level of the trainer and be pre-programmed to control a workout. Some can even display video of an animated figure or some real life video to provide a distraction from the effort and help to pass the time.

Of course the most obvious answer is simply to HTFU.

Adaptation is the last of the four key issues. Since there are differences in riding on a trainer to riding outdoors, some of which have been discussed already, then it stands to reason it will take some time for the body to adapt to training under different conditions. If you only ride the trainer occasionally, then you may never fully adapt to being able to generate power similar to outdoor riding.

However, if you ride on a trainer regularly and with sufficient volume, and you address the other three main factors, then you will adapt and improve your ability to produce power indoors and the gap to outdoor power will typically narrow.